The human immunodeficiency virus type 1 (HIV-1)/AIDS epidemic in southern Brazil is undergoing a major transformation at both the molecular and epidemiological levels. Brazil has long been host to HIV-1 subtypes B and F, but within the last ten years, a unique, Brazilian-specific subtype C has appeared [1]. Recent studies have estimated that the prevalence of HIV-1 subtype C is increasing substantially in southern Brazil [2]. Viral and host factors contributing to the increase of HIV subtype C are not known, but may have major importance for HIV vaccine development and prevention strategies relevant to the region. There are two principal hypotheses behind the proposed research: First, that patients with incident subtype B and subtype C HIV infections can be reliably identified and recruited from clinics in southern Brazil by using novel procedures for HIV testing; and Second, that sufficient numbers of very early (antibodynegative) viral specimens can be collected from these patients to permit rigorous study of viral factors related to transmission potential and vaccine development. This exploratory proposal intends to identify and study a cohort of Brazilian patients with acute HIV infection (AHI) in southern Brazil and to preliminarily characterize actively circulating subtype B and C HIV strains with respect to replication capacity, co-receptor usage and sequence changes that may occur in HIV envelope shortly after HIV transmission. The proposed pilot study also intends to strengthen independent research capacity and further the development of local scientific expertise. The Southern Brazilian site is unique for testing this hypothesis for several reasons: First, it is an area with high prevalence rates of HIV/AIDS, increasingly dominated by a virus with unique molecular characteristics; Second, it has a strong government-supported program to prevent and treat HIV infection; Third, it has an already growing research infrastructure, and a committed interdisciplinary group of people interested in the development of comprehensive AIDS research through strong partnerships. [unreadable] [unreadable]